Jenny Nimmo was born in Windsor, Berkshire, England and educated at boarding schools in Kent and Surrey from the age of six until the age of sixteen, when she ran away from school to become a drama student/assistant stage manager with Theater South East. She graduated and acted in repertory theater in various towns and cities: Eastbourne, Tunbridge Wells, Brighton, Hastings, and Bexhill.

She left Britain to teach English to three Italian boys in Almafi, Italy. On her return, she joined the BBC, first as a picture researcher, then as an assistant floor manager, studio manager (news) then finally a director/adaptor with Jackanory (a BBC storytelling program for children). She left BBC to marry a Welsh artist David Wynn Millward and went to live in Wales in her husband's family home. They live in a very old converted watermill, and the river is constantly threatening to break in, as it has done several times in the past, most dramatically on her youngest child's first birthday. During the summer they run a residential school of art, and she has to move her office, put down tools (type-writer and pencil, and don an apron and cook! They have three grown-up children, Myfawny, Ianto, and Gwenwyfar.